Optical waveguides in general either are manufactured in the form of cladded optical fibers or in the form of optical material that is patterned onto a wafer so as to provide, for instance, an arrayed optical waveguide that is used in separating out different wavelengths. In such an application, incoming light is made to travel down through one waveguide section or another, depending on the wavelength of the incident light.
With respect to optical waveguides formed by layers deposited on a silicon wafer, the layers that are deposited are relatively thick and are annealed on the wafer using a relatively high temperature. In the process, the wafer becomes warped such that the wafer is bowed or dish-shaped. When one wants to use photolithography to pattern deposited layers, either the wafer is warped such that proper registration of the photolithography pattern cannot be obtained or, in extreme cases, the wafer itself is so warped that it cannot be properly secured to the vacuum chuck used by the stepper.
In either case where the photolithography does not provide the appropriate patterning of the waveguide on the wafer, or in the case where the entire wafer is rejected by the stepper, it is important to be able to find a technique that can successfully accommodate the deposition of a relatively thick 1.5 micron thickness of, for instance, silicon oxynitride, which forms the waveguide material on the wafer, and do this without inducing wafer distortions. If wafer warping is not appropriately controlled, either the waveguide will not function properly to provide total internal reflection or support TEM modes, or the wafers cannot be loaded into the stepper.
In order to maintain the tight tolerances required by optical waveguides formed on a wafer, one must first start with a flat wafer surface so that one can photoprocess across the whole wafer and still have all surfaces of the wafer be in the same focal plane.
In order to establish such a surface, the wafer is polished flat so that when it is transported to the fabrication section and processing is started to add levels or films to the wafer, the wafer surface must be flat. However, thermally-induced stress in the wafer causes the wafer to dish. The result is that the wafer is not as flat as it needs to be. Because of the stress-induced shape of the wafer, one obtains focus variations and critical dimension variations across the wafer. This in turn translates to waveguides that have critical dimensional errors.
It will be appreciated that the purpose of the waveguide is to provide total internal reflection of the incoming light so that it reflects down the waveguide and maintains the appropriate mode of propagation. While it is possible to glue optical fibers onto a wafer to provide an optical array, it is preferable to simply lay down an optically transparent film on the wafer and then etch structures in the wafer that constitute fiber-like waveguides. While the waveguides are square as opposed to round, assuming that the waveguides are properly made, they functioned equally as well as optical fibers.
If on the other hand there are refractive index changes from the stress-induced bowing process when a relatively thick later is annealed to a wafer, the critical dimensions of the waveguide can deleteriously affect the channeling of the light.